The manufacture of fibers and films from cellulose by the viscose process has been known for more than a hundred years. Even today, almost all cellulose-based fibers are manufactured by the viscose method. It is a known method, by which various properties of the final product are achieved by varying the material and process parameters. However, the viscose method involves significant drawbacks: the preparation of the spinning solution includes laborious work stages, the carbon disulphide used for the dissolution is toxic, inflammable and combustible, and it is difficult to recover. Furthermore, some of the carbon disulphide is decomposed to hydrogen sulphide, which is also toxic and explosive. In addition, the viscose solution is an unstable product which cannot be stored as an intermediate product, but all the steps of the manufacture must be taken without a delay from the beginning to the end, often keeping the pulp at a low temperature.
Several attempts are known to replace the viscose method with a more ecological method. The most promising one has been the conversion of cellulose to cellulose carbamate by means of urea (see, for example, D. Klemm et al., Comprehensive Cellulose Chemistry, Wile-VCH 1998). One of the first attempts to manufacture cellulose carbamate is presented in U.S. Pat. No. 2,134,825. Finnish patents FI 112795 and FI 112869, and the corresponding international patent application WO 03/064476, disclose a novel and effective method for preparing cellulose carbamate (abbreviated CCA). In this method, the chemicals are dosed in atomized form into ground cellulose in a fluidized bed type mixer, and the homogenization of the pulp and the penetration of the chemicals into the fibers are effected by a mechanical working device, such as a sieve plate press. The final reaction takes place in an oven or the like, in which the temperature is raised above the melting point of urea, 133° C.
Cellulose carbamate is alkali soluble at a substitution degree of 0.1 to 0.3. As early as in the above-mentioned old US patent, it has been disclosed that the dissolution is advantageously made at a reduced temperature. Typically, the dissolution temperature of −5° C. has been applied. The preparation of the solution has also been discussed in U.S. Pat. No. 6,590,095. It presents a two-step dissolving technique, in which the first step is taken by using an NaOH solution that is stronger than the final solution, and in the second step, the solution is diluted with water and/or an aqueous NaOH solution. The presented dissolving technique, as all known CCA dissolving techniques, is characterized by a long mixing time (>1 hour). Furthermore, according to the patent, the preparation of the solution is independent of the DP level (degree of polymerization) of the carbamate.